The principle of induction heating for cooking purposes has been known for many decades. It is usually based on one or several induction coils of flat, spiral shape, each one being inserted in a resonance circuit, and forming the primary winding of a transformer. The metallic cooking utensil, such as a pan, forms the secondary winding having essentially a single turn. In operation, the resonance circuit is generally driven off-resonance at a frequency above the audible range. The cooking utensil is heated by eddy currents induced by the time varying magnetic field associated with the induction current, and by magnetic domain switching when magnetic materials are being used.
The power supplied to a pan can be adjusted by positioning the pan at an appropriate position above an induction coil, or, alternatively, by adjusting the power fed into the resonance circuit. This may be realized either in a frequency control mode, wherein the frequency of the driver circuit is changed, or in a pulse control mode, wherein the width of current pulses supplied to the induction coils is changed at a constant frequency (see, e.g. the European patent application no. EP 1 494 505 to Weder, the entire disclosure of which is hereby incorporated by reference).
It has been recognized long ago that it is advantageous to use asymmetric induction coils, as these allow uniform heating of a cooking utensil to various degrees, depending on its location on the stove top (see for example U.S. Pat. No. 3,843,857 to Cunningham, the entire disclosure of which is hereby incorporated by reference). Shifting a pan to the edge of an induction coil and beyond has, however, the effect of causing large phase shifts in the resonance circuit and large induction currents, while the eddy currents in the pan are being reduced. This may cause serious problems of overheating for the transistors of the power supply, which need to switch unnecessarily high currents in comparison to a central position of the cooking utensil.
A similar problem may arise to an even greater extent when improper pans with a low magnetic permeability are being used. In this case the phase shift may be very large at any positioning of the pan relative to the underlying induction coil. It is therefore important to determine the quality of a cooking utensil before applying excessive power to the resonance circuit. One way of checking the quality of a pan is to measure the induction current and the mains current for at least two power settings, and to compare their ratio with predefined set-point values (see for example the International patent application no. WO 2008/055370 to Meier, the entire disclosure of which is hereby incorporated by reference). The ratio depends, however, not only on the material, but also on the size and exact location of the pan. This is why this method may be bothersome to apply.
Furthermore, the use of an inappropriate cooking utensil may lead to equally large phase shifts and coil currents as a badly positioned one, with similar undesired consequences for the driving circuitry.
What is needed therefore is a means of protecting critical circuit elements from overheating caused by the use of poor quality cooking utensils or poor or improper positioning of the cooking utensil on the coil.